FOUS30 KWBC 120803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Mon May 12 2025

Day 1
Valid 12Z Mon May 12 2025 - 12Z Tue May 13 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEAST FLORIDA...

A deep upper low will be slowly moving north towards the Tennessee
Valley, dragging a surface low pressure system north with it. The 
trailing cold front will gradually push towards the Atlantic Coast 
while the warm front lifts north into the southern mid-Atlantic. 
This system will direct a stream of deep anomalous moisture into 
the Southeast and Mid-Atlantic that will support widespread showers
and storms with heavy rain potential across these regions. PWATs 
are expected to surge to over 1.5 inches from Virginia to Georgia 
and over 2 inches over the Florida Peninsula, which will certainly 
provide the needed moisture for elevated rain rates in stronger convection.

A Moderate Risk area has been introduced for the urban corridor of
Southeast Florida where heavy rain may result in significant 
flooding impacts. The synoptic setup is favorable for a heavy rain 
event over South Florida with deep moisture and southerly winds 
shifting to southwesterly ahead of an approaching cold front. Waves
of showers and storms are expected throughout the period, and 
there will be plenty of moisture and instability to support high 
rain rates in excess of 2 inches per hour. CAMs are showing the 
potential for 3-4, locally up to seven, inches of rain possible 
today. Yesterday's 18Z HREF was showing 60-70% probability of 24 
hour QPF exceeding 8 inches, but the 00Z HREF dropped probabilities
of 8 inches in 24 hours down to 40-50%, indicating some 
uncertainty in how high the max QPF amounts may be. Regardless of 
the exact amount, flooding impacts are likely for the urban 
corridor of Southeast Florida. 

To the north, strong south-southeasterly mean flow will focus QPF 
maxes along the Carolina Coast and along upslope regions of the 
southern Appalachians. CAMs have consistently shown 2-4 inches of
precipitation, with locally higher amounts possible, over the 24 
hour period in these areas. There was discussion about potentially 
upgrading to a Moderate Risk for sensitive areas in the 
Appalachians, but it was decided to remain at a high-end Slight 
Risk given a lack of instability and limited rain rates. Rain rates
could periodically exceed 1 inch per hour, which would support 
isolated to scattered flash flooding concerns for the western 
Carolinas and Southwestern Virginia, but most likely would not 
support a risk higher than Slight. The higher rain rates (1-2 
inches per hour) are expected to be in convection near the coasts 
where instability will be higher. 

To cover these flooding threats, a Slight Risk area extends from
Virginia to the Florida peninsula. There will also be localized
heavy rain potential across the Tennessee and Mid-Mississippi
Valleys, Mississippi, and Alabama. There will be decent instability
in the vicinity of the surface low to produce convection with 1
inch per hour rain rates. Given that soils in these regions are
saturated from recent heavy rains, isolated flooding concerns will
be possible, and therefore a Marginal Risk of excessive rainfall is
in place. 

Dolan


Day 2
Valid 12Z Tue May 13 2025 - 12Z Wed May 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC...

The slow-moving upper low and occluded surface low will continue 
to move north, and showers and storms will focus across the Mid-
Atlantic on Tuesday. Deep moisture ahead of the system will push 
PWAT values above 1.5 inches, and there will be enough instability 
(MUCAPE > 1500 J/kg) across portions of North Carolina and Virginia
to support high rain rates (1-2 inch per hour), which may rival 1 
and 3 hr FFGs. Upslope flow along the Appalachians from southern 
Pennsylvania through Virginia will also help to enhance rainfall, 
with generally 1.5-2.5 inches of rain expected across this area 
with locally higher amounts possible. After heavy rain on Monday, 
these areas may be more sensitive to flooding given higher soil 
moisture, and isolated to scattered flash flooding concerns are 
expected. This will warrant a continuation of the Slight Risk area 
from northeastern North Carolina through Virginia and portions of 
West Virginia, Maryland, and southern Pennsylvania. 

Isolated flash flooding may also be possible for portions of the
Ohio Valley and Appalachians. Moisture wrapping around the northern
side of the low pressure system combined with sufficient
instability near the surface low will support scattered convection
with relatively slow storm motion. Therefore, a Marginal Risk is in
place for these regions. 

Dolan


Day 3
Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND MID-ATLANTIC...

...Northern Plains...

A deep upper level trough will push across the Northwest to the 
Plains on Wednesday, resulting in a strengthening low pressure 
system in the northern Plains. Stratiform rain with likely some 
embedded convection on the northern and western side of the low are
forecast to produce 0.5-1 inches of rain, with locally higher 
amounts possible, in eastern Montana and northeastern Wyoming. The 
main threat will be due to the persistent nature of the 
precipitation in this area, starting up Tuesday night and 
continuing through Wednesday into Wednesday night. Rainfall totals 
will likely come close to the 6 hour FFGs in eastern Montana. 

For the Dakotas, convective precipitation is forecast to develop 
ahead of the low along a strengthening frontal boundary. Models are
in good agreement on the axis of heaviest precipitation setting up
over central North Dakota and eastern South Dakota. PWAT values 
are expected to be at least 2 standard deviations above normal 
(1-1.5 inches), and there should be enough instability (MUCAPE > 
1500 J/kg) during the day on Wednesday to support at least an 
isolated flash flood threat. 

...Mid-Atlantic...

Soils are expected to be thoroughly saturated across the Mid-
Atlantic after heavy rain on Monday and Tuesday, which may create a
low end flood threat with lingering showers on Wednesday. The 
slow-moving surface low is forecast to track right over the Mid-
Atlantic with scattered precipitation, and there should be enough 
instability near the low to support some convection capable of 
producing locally heavy rain. Therefore, a Marginal Risk area was 
introduced for portions of the Mid-Atlantic.

Dolan

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